Fix not affected variables.

Variables which is not exported must be exported explicitly.

----

I needed to set a variable 'PERL5LIB' in `release` script of my custom buildpack.
After `builder`, I got config_vars following.
```bash
# in buildstep based image
$ cat /app/.profile.d/config_vars 
PATH=local/bin:vendor/bin:/usr/local/bin:/usr/bin:/bin
PERL5LIB=local/lib/perl5

$ /exec env
PATH=local/bin:vendor/bin:/usr/local/bin:/usr/bin:/bin
```
but, /start and /exec cannot recognize PERL5LIB.

It should be like following.
```bash
# in buildstep based image
$ cat /app/.profile.d/config_vars 
export PATH=local/bin:vendor/bin:/usr/local/bin:/usr/bin:/bin
export PERL5LIB=local/lib/perl5

$ /exec env
PATH=local/bin:vendor/bin:/usr/local/bin:/usr/bin:/bin
PERL5LIB=local/lib/perl5
```